The scenario might have seemed unlikely: prominent Muslims and Jews from the United States, trekking across the Atlantic in mournful, spiritual solidarity to visit two Nazi concentration camps. Together.

The trip to Dachau and Auschwitz was meant to combat the rise in Holocaust denial that has popped up in various Muslim and non-Muslim circles around the world--and online--in recent years.

"The best way to convince someone about the truth of something is to let them see it for themselves and experience it for themselves," said Rabbi Jack Bemporad of the Center for Interreligious Understanding in Carlstadt, who organized the trip.

"I feel that it was important to take Muslim leaders who have a really significant following in the American-Muslim community."...